1. Field of the Invention
The present invention relates to a method of electron beam exposure, and more specifically to a method of electron beam exposure which automatically adjusts the focal distance in the system of electron beam lenses and which also automatically adjusts the exposure conditions of the electron beam exposure. The method of electron beam exposure of the present invention can be applied during a step of electron beam exposure in the production of, for example, integrated circuit semiconductor devices.
2. Description of the Prior Art
A beam adjusting means for correction thereof has so far been employed in order to accurately impinge the beam onto the surface of a specimen, or to accurately perform beam scanning during a step of electron beam exposure. For example, reference can be made to the following literature:
J. L. Mauer et al., "Electron Optics of an Electron-beam Lithographic System", IBM Journal of Research and Development, November, 1977. PA1 H. Engelke et al., "Correction of Nonlinear Deflection Distortion in a Direct Exposure Electron-beam System", IBM Journal of Research and Development, November, 1977. PA1 (1) Field size for determining the scanning range of the electron beam. PA1 (2) Rectangularity (orthogonality) given by an angle between the direction of longitudinal scanning and the direction of lateral scanning of the electron beam. PA1 (3) The beam feedback value which is given by a value K.sub.bf having the relationship K.sub.bf =K.sub.dev /.alpha. with a value K.sub.dev. When the spot of the beam deviates by length .DELTA.x on the specimen due to the current I of the shifting coil 53, the relationship between .DELTA.x and I is expressed as I=K.sub.dev .multidot..DELTA.x. In the former equation, .alpha. is a constant determined mainly by the gain of the amplifier 89.
According to the above-mentioned conventional methods, a focal distance in the electron beam lens system and the exposure conditions for exposing with the electron beam, are set prior to effecting the electron beam exposure. However, once such conditions have been set, it is not allowed to adjust the setting conditions until the electron beam exposure is finished over the whole specimen. In this case, the exposure conditions for electron beam exposure consist of the following three condition requirements:
However, usually the semiconductor wafer used as the exposure sample is not strictly flat but is warped to some extent, or is tongued and grooved. Therefore, to expose such a semiconductor wafer to the electron beam means that resist patterns formed on the specimen undergo variations caused by the unevenness of the surface while the specimen is being exposed. Therefore, the focal distance and the electron beam exposure conditions must be constantly corrected if the specimen is to be precisely exposed. If these corrections are not made, the size and position of a resist pattern formed by the electron beam exposure become incorrect. Therefore, when a large area which requires a long exposure time is exposed by using electron beam exposure, the precision of the formed patterns is decreased. Especially, in the case of composite patterns due to the switching of the stages, the formed patterns lack precision. Consequently, it becomes difficult to form a pattern of large areas while maintaining precision. Moreover, according to conventional methods, the beam was not effectively controlled when the specimens were exposed to the electron beam, and it was difficult to suitably expose the specimens to the electron beam. The present invention is proposed in order to eliminate the above-mentioned problems inherent in the conventional methods.